Use of the ILS does not require all of these components, however.
For example, an instrument approach on the ILS system may be made with
only localizer and marker beacon receivers (sometimes called a "localizer
approach"); with only localizer and ADF receivers; or with an ADF receiver
only, using the locator as a primary approach aid. The authorized landing
minimums will of course vary according to the ground and airborne equipment
available and operating properly.

Localizer Receiver. The typical light-plane VOR receiver is also
a localizer receiver with common tuning and indicating equipment. Some
receivers have separate function selector switches. Otherwise, tuning of
VOR and localizer frequencies is accomplished with the same knobs and switches,
and the CDI indicates "on course" as it does on a VOR radial.

Figure 7-31. Standard FAA 2-bar VASI.

Glide Slope Receiver. Though some glide slope receivers are tuned
separately, in a typical installation the glide slope is tuned automatically
to the proper frequency when the localizer is tuned in. Each of the 40
localizer channels in the 108.10 to 111.95 MHz band is paired with a corresponding
glide slope frequency of the 40 UHF glide slope channels available (see
AIM). Although all of these 40 frequency pairs are not presently being
utilized, they will be in the future.

Localizer/Glide Slope Indicator. When the localizer indicator
also includes a glide-slope needle, the instrument is often called a crosspointer
indicator. The crossed horizontal (glide-slope) and vertical (localizer)
needles are free to move through standard 5-dot deflections to indicate
position on the localizer course and glide path. See Figure 7-32.

The localizer needle indicates, by deflection, whether the aircraft
is right or left of the localizer centerline, regardless of the position
or heading of the aircraft. Rotation of the omni bearing selector has no
effect on the operation of the localizer needle. When the aircraft is inbound
on the front course or outbound on the back course, the needle is deflected
toward the on-course (A, Fig. 7-33) and you turn toward the needle to correct
your track. Conversely, when the aircraft is inbound on the back course
or outbound on the front course, you turn away from the direction of needle
deflection to reach the center of the localizer course (B, Fig. 7-33).
With an ADF tuned to the outer compass locator, orientation on the localizer
course is simplified (C, Fig. 7-33). The problem of orientation with respect
to both the localizer course and VOR radials will be discussed in Chapter
VIII, "Using the Navigation Instruments."

The localizer course is very narrow, normally 5°. This results
in high needle sensitivity. With this course width, a full-scale deflection
shows when the aircraft is 2.5° to either side of the centerline. This
sensitivity permits accurate orientation to the landing runway. With no
more than 1/4-scale deflection maintained, your aircraft will be aligned
with the runway. High needle sensitivity also tends to encourage overcontrolling,
until you learn to apply correct basic flying techniques for smooth control
of the aircraft.

Deflection of the glide slope needle indicates the position of
the aircraft with respect to the glide path (Fig. 7-34). When the aircraft
is above the glide path, the needle is deflected downward. When the aircraft
is below the glide path, the needle is deflected upward.

When the aircraft is on the glide path, the needle is horizontal,
overlying the reference dots. Since the glide path is much sharper than
the localizer course (approximately 1.4° from full "up" to full down"
deflection), the needle is very sensitive to displacement of the aircraft
from on-path alignment. With the proper rate of descent established on
glide slope interception, very small corrections keep the aircraft aligned.

The localizer and glide slope warning flags disappear from view
on the indicator when sufficient voltage is received to actuate the needles.
The flags show when an unstable signal or receiver malfunction occurs.

ILS flight procedures will be discussed in Chapter VIII, "Using
the Navigation Instruments." ILS approach procedure charts are described
in Chapter X, "The Federal Airways System and Controlled Airspace." Additional
information on the FAA ILS system appears In the Airman's Information Manual
under the heading, AIR NAVIGATION RADIO AIDS.